1. Field of the Invention (Technical Field)
The present invention relates to an irrigation sprinkler control system using relative humidity ("RH") as a control parameter, and a method for determining the optimal RH set-point for comparison with the ambient RH.
2. Background of the Invention
Supplemental irrigation is used in many regions of the country to supply water for plant growth. Typically the driest periods guide the fixed quantity of supplemental water that is applied for each regularly scheduled irrigation sequence. However, intermittent periods of relatively high precipitation and high-humidity reduce or eliminate the need for supplemental watering. As a result an irrigation system that curtails supplemental watering during these periods can reduce water consumption. One such system includes a relatively expensive computer-controlled irrigation programmer that is interactively coupled to network services that provide real-time weather data. Thus, if the weather service predicts precipitation the computer will be instructed to curtail the irrigation sequence for a scheduled period. Other less expensive systems are continually sought for large numbers of lesser-volume users, for example, for home-use applications.
Supplemental irrigation programs regularly apply fixed volumes of water sufficient to meet the plant or turf requirements for the neediest stages of a given time period. For economical irrigation during this period, the applied water should be curtailed for intervals with low need. Plant watering needs are represented by a calculated parameter called evapotranspiration ("ET"). A daily ET value is generated by the service provider. This ET value considers four different weather parameters that are integrated over the 24-hour period. Daily variations of the ET value are the industry-accepted basis for planning and scheduling irrigation. Low ET values (low plant-water needs) generally coincide with interludes of high RH and low temperature. The demonstrated correlation of the daily variations of ET and RH provides the basis for using RH as an irrigation-control parameter. This correlation allows the humidistat to effect considerable water savings if properly used in the irrigation control system.
However, ambient RH is constantly changing. For high-altitude desert regions, the daily or diurnal RH variation can be more than 70%. Selecting the time of day for measuring the ambient RH and comparing that value with the irrigation control RH set-point is critical for maximizing the correlation of the RH measurement with plant water needs. Historical daily weather data is widely accessible, and daily ET data is available for nearly all regions with an agricultural base as well as for some metropolitan regions. Weather and ET data from these sources indicate general behavior patterns that are common to identifiable geographical regions.
Caprio, U.S. Pat. No. 5,853,122 is a relatively inexpensive device that uses RH as the control parameter for selectively curtailing supplemental irrigation cycles. A humidistat in the device compares the ambient RH with a pre-selected RH set-point. If the ambient RH is greater than the RH set-point irrigation will be curtailed. Conversely, if the ambient RH is less than the RH set-point irrigation, the irrigation sequence will begin. However, a major drawback of the Caprio device is that the humidistat is constantly measuring the ambient RH and controlling the irrigation system. For example, if the ambient RH rises above the RH set-point during an irrigation period, the irrigation will be halted. If such an event is repeated from day to day the initial programmed zones will receive more water than the latter programmed zones.
Another drawback to the Caprio system is that it cannot be programmed to measure ambient H and compare that value with the RH set-point at one particular time of the day, store the information as to whether or not to irrigate, and then irrigate at an alternative time of the day. Generally, it is highly advantageous to irrigate in the early morning, when loss due to evaporation is lowest. Unfortunately, in the early morning the ambient RH is generally at or near its maximum value, and, except for high arid regions, there often are extended periods with very little change from day to day in the RH, which is the control parameter. Therefore, for most regions reliance upon the daily maximum RH value does not provide a suitable control parameter. In contrast, ET data, which is a measure of actual plant needs, provides a more accurate means to determine if supplemental irrigation is needed. ET data also correlates well with the daily minimum RH values, which are generally available in the early evening, some twelve hours prior to the optimal irrigation period.
The present invention solves both of the problems encountered with the Caprio device by making the ambient RH measurement at the time of day where it shows the strongest correlation with the applied-water needs indicated by historical ET data coupled with other meteorological data. The irrigation control condition, i.e., water or don't water, is determined by the RH measurement, and remains active for the remainder of the irrigation period, usually one day. Therefore, one can measure the ambient RH and compare that value with the RH set-point at 6:00 PM and, if the condition is met, begin irrigation at 6:00 AM the next day regardless of what the ambient RH is at 6:00 AM. Also, because the present invention does not interrupt irrigation in real time, irrigation will not be halted in the middle of the programmed irrigation sequence if the ambient RH drops below the RH set-point. Therefore, all irrigation zones are given their respective amounts of water as programmed.